1. Field of the Invention
The invention relates generally to information carriers with diffraction structures and can be applied in particular to an information carrier having at least one diffraction pattern which has diffraction structures formed from microscopically fine relief structures and which, upon being illuminated with coherent light, produces in two spatially separate directions first and second images of an object which images can be rendered visible on a screen or analysed by means of photodetectors.
Such information carriers are preferably suitable for use as security elements for documents of all kinds as they are provided with items of information which can be discerned by eye or by photodetectors only under lighting with coherent light.
In the case of Fourier, Fresnel and Fraunhofer holograms the holographically stored object is not visible under normal, that is to say substantially incoherent, lighting conditions. However the object can be reconstructed as a real or as a virtual image when the hologram is lit with coherent light. In the case of white-light holograms such as for example rainbow holograms or with certain volume holograms on the other hand the stored object is also discernible under normal lighting conditions. An overview of the field of holography is to be found in the book "Handbook of Optical Holography" which was published by Academic press, Inc. in 1979.
2. Description of the prior Art
German laid-open application (DE-OS) No. 42 37 415, discloses an information carrier with a Fourier transformation hologram which, upon being lit with a laser beam, produces an image of a holographically recorded, two-dimensional object which can be made visible on a suitably arranged screen. To produce a visible image of the object on the screen, it is sufficient if the laser beam lights only a small surface portion of the hologram. More specifically, Fourier holograms have the advantageous properties that the image which is reconstructed in that way is independent of translatory movements of the information carrier and also has a great depth of focus. It is known from the section about Fourier holography in the above-quoted handbook that reconstruction involves the production not of a single direct image of the object but also a second conjugate image. With a perpendicular direction of lighting the two images are arranged in point-symmetrical relationship with respect to the axis of the laser beam and are of equal brightness.
An example of the two images of an object representing the word "CSIRO" which are produced upon coherent lighting of a Fourier hologram is shown photographically in the book "Optical Holography" by P. Hariharan in the series "Cambridge Studies in Modern Optics: 2" published by Cambridge University press (ISBN 0 521 31163 2).
The combination of a Fourier hologram and a hologram which is visible under normal lighting conditions is known from the Eurocheck card in the case of which the letters "ABNH" can be rendered visible on a screen upon being lit with coherent light, for example with a laser.
Fourier holograms may be for example in the form of microscopically fine relief structures, also referred to as surface reliefs, so that they can be inexpensively duplicated by embossing into a thermoplastic layer or by moulding into an UV-hardenable layer. Upon being lit with a laser emitting visible light, two images of the object are visible with the same levels of brightness on a suitably arranged screen.
Instead of Fourier holography it is also possible to use computer-generated holograms for producing a predetermined image. What is suitable for that purpose is in particular the kinoform which can be produced in the form of a surface relief.